The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Sound amplifying devices for amplifying the sounds emitted from a motor vehicle engine are sometimes referred to as “symposers”. Symposers are used in motor vehicle applications to enhance and/or modify the sound produced by a vehicle engine as the engine is operating. With present day vehicle manufacturers placing a heavy emphasis on sound proofing the cabins of their vehicles, this has resulted in significantly attenuating the engine sound within the vehicle's cabin. However, in certain instances, and sometimes with certain vehicles such as sports cars, sedans, sport SUVs or sport trucks, it is desirable to provide a clearly audible engine sound within the cabin when the vehicle is accelerating or otherwise operating under a heavy load. With sports cars and other performance/racing inspired vehicles, this is especially so, as the driver typically appreciates and enjoys the engine sound emitted within the cabin under hard acceleration.
Conversely, the need for audible engine sounds within the cabin is not necessarily as desirable while the vehicle is cruising with its engine operating at a relatively low RPM. So for example, while cruising down a highway in fifth or sixth gear, with the engine operating at, for example 2000 RPM or lower, there is a desire to attenuate the engine sound heard within the cabin by the vehicle operator. This enhances the operator's ability to listen to dialogue or music provided by the vehicle's audio system and reduces the need to raise the volume to overcome the sound of the engine.
Previously developed symposers have typically relied on the use of internally mounted valves that modify and/or restrict a flow path for intake airflow through the symposer. The valve is typically controlled in accordance with engine RPM. In this manner the amplitude of sound produced by the symposer can be controlled at least somewhat in accordance with the changing engine RPM during acceleration. However, since the length of the flow duct associated with such a prior developed symposer does not change, and nor does the internal volume within the duct change, such systems are typically limited to operating within a relatively narrow frequency range. Moreover, such “static” duct constructions are not amenable to being tuned to emphasize or highlight engine sounds at specific, desired frequencies. As a result, the amplification achieved may not necessarily be achieved over a wide frequency spectrum or a desired frequency spectrum. Put differently, such prior developed systems may only be able to provide a desired level of amplification of the engine sounds emitted while the engine is operating within a narrow frequency band. And since the frequency of the emitted sound can change from a lower frequency to a significantly higher frequency as engine RPM increases, this means that a good portion of the frequency spectrum associated with the sounds made by an accelerating engine may not be detected and amplified sufficiently.